Increasing demands (e.g., US07, Euro5) on modern diesel engines with respect to their emissions, in addition to requiring new systems for exhaust-gas treatment, also require the development of new combustion processes for reducing emissions within the engine. So-called (partial-) homogeneous combustion processes (also known as (p)HCCI processes) represent one potential possibility in this regard. One characteristic these processes share in common is a sharply increased exhaust-gas recirculation (EGR) rate compared to conventional combustion processes. For design reasons, already during steady-state operation, this leads to different filling compositions (ratio of inert gas/fresh air) specific to each cylinder, and as a result of manufacturing tolerances and aging effects of the engine over its service life, leads both to combustions proceeding very differently specific to each cylinder, and to sharp sample strews. This, in turn, leads to very different pollutant and noise emissions specific to each cylinder, which is unwanted.
On the other hand, combustions proceeding differently specific to each cylinder can be detected by ascertaining the combustion position and the mean indicated torque, and adjusted, if necessary. In this respect, ascertaining and controlling these combustion parameters for equalizing the cylinders represents one possibility for improving the combustion.
There are conventional methods for determining cylinder-specific combustion features from the cylinder-pressure signal and the structure-borne sound signal, which are used in particular for combustion processes having a high EGR rate (e.g., pHCCI combustion processes).
In principle, the combustion position can be determined robustly by cylinder-pressure indication; however, the additional costs for the production use of cylinder-pressure sensors are so high that, particularly in the case of smaller engines (e.g., 4 cylinder) and high piece numbers, they must be judged as critical. Therefore, an object of the present invention is to ascertain cylinder-specific combustion features in a manner that is more cost-effective than in methods heretofore.